When coordinating RF transmitters it is important to take into account the phenomenon of intermodulation (IM) distortion. Intermodulation distortion is caused by non-linear amplifiers and signal processing used in most audio hardware. Intermodulation distortion between two or more frequencies will form additional frequency signals (intermodulation products). These new signals occur at the sum and difference frequencies of the original signals and at multiples of those sum and difference signals. If intermodulation products fall within the bandwidth of a receiver, intermodulation interference may occur.
Those configuring wireless equipment for a live entertainment event, typically referred to as “RF coordinators,” are faced with many challenges. Typically, for a concert or a sporting event, multiple systems independently operating in parallel communicate by means of wireless signals. These systems may collectively use dozens of wireless channels, each typically having a width on the order of 25 KHz.
In a live concert, there are wireless channels assigned to lead singers, various accompanying instruments, backup singers, in-ear monitors worn by musicians, and two-way radio communications between people coordinating the performance. In addition, if there is news coverage of the event the news reporters will have wireless communication devices, also competing for clean spectrum space.
Sporting events also have a pall mall assortment of sound sources that must be transmitted wirelessly to receivers. Coaches and assistant coaches are connected by wireless units. News crews are equipped with wireless communication devices.
Moreover, clean spectrum space is not necessarily easy to find, as the 470 MHZ to 700 MHZ spectrum typically used for live events is, for the most part, shared with UHF TV stations. Because of this, even the same concert, with the same set of performers and instruments, cannot have the same wireless channel assignments from one city to the next on a multi-city tour.
Avoiding interference caused by intermodulation products is a critical issue in this environment. There are software tools available to assist an RF coordinator in assigning channels to their wireless equipment. The current state of the art is for the software tool to perform an intermodulation analysis and compute a frequency set. The resultant frequency set includes a list of frequencies that are guaranteed to be “intermodulation-compatible”—that is, the intermodulation products computed from the frequencies in the frequency set are guaranteed to be a specified distance removed from each frequency in the set. Sometimes, when intermodulation analysis is performed using strict criteria for frequency survival then the resultant frequency set is too small to accommodate all the audio gear that requires channel assignments. In this case the intermodulation analysis can be repeated using less stringent frequency survival criteria, and this results in a frequency set that contains more members but which is also less reliable than one computed using stricter criteria. Typically, all frequencies in a frequency set are treated as being equally reliable and the RF coordinator typically assigns them, in no particular order, to their wireless equipment. To be more exact, an RF coordinator assigns channels to their wireless equipment, where a channel is a frequency band (typically 25 KHz wide) whose center frequency is a member of the frequency set.